Increasing requirements are seen recently for simplification of the mounting system, miniaturization, high reliability and high performance of the communication appliances, domestic appliances, electronic computers, and sounding devices, and similar requirements are made of the printed circuit boards incorporated in such appliances.
Among printed circuit boards, flexible printed circuit boards can meet these requirements as they are light in weight, compact in structure, and capable of bending and three-dimensional wiring, and they are recently, therefore, prevalently used as printed circuit boards for a variety of appliances.
As such flexible printed circuit base boards, there have been used the so-called flexible copper-clad laminates obtained by bonding an electrically insulating film such as polyester resin film, polyimide resin film, fluorocarbon resin film, etc., to a copper foil through an adhesive or by bonding to a copper foil a prepreg prepared by impregnating a fibrous base material such as glass cloth, glass non-woven fabric or the like with a heat-resistant resin, such as epoxy resin or the like. However, in the case of bonding an electrically insulating film to a metal foil through an adhesive, the product quality is greatly affected by not only the properties of the film per se but also the properties of the adhesive used, and particularly in the case where the product is based on a film excellent in heat-resistance and dimensional stability, such as polyimide film, the innate properties of the film such as heat-resistance and the like, are attenuated by the adhesive and not utilized to produce the best effect. On the other hand, the copper-clad laminate in which the fibrous base material is used is insufficient in flexibility, such as folding endurance, and often breaks when used in three-dimensional wiring as a printed circuit board, whereby troubles are often caused. There is also known a method of bonding a plastic film, a resin powder or the like to a metal foil by melt-adhesion without using any adhesive, but the films or powders with high heat-resistance have themselves a high melting point and are hard to melt-adhere, so that there are many limitations, such as a specific treatment or a very high temperature pressing equipment is required. Therefore, it is not easy and is very costly to carry out said method on an industrial scale.
On the other hand, when an electric insulating resin is coated in a wet state on the surface of a metal foil and dried to form a coating, and the thus obtained coating per se is used as an electric insulating layer and a support for a flexible printed circuit, the resulting laminate has various advantages as mentioned below as compared with the above-mentioned conventional materials and becomes a very commercially attractive laminate:
(1) Because of use of no adhesive, the properties of the resin per se are retained in their entirety in printed circuit boards.
(2) It becomes unnecessary to subject a film along with a metal foil to pressing step, and hence, the productivity is increased.
(3) The control of the thickness of the coating as an insulating layer and a support can freely be effected to enable the manufacture of a very thin base board.
(4) Continuous production is facilitated. A solution casting method is known for the formation of such a coating, and a varnish of a resin, such as epoxy resin, urethane resin, alkyd resin or the like is known to be able to form a coating on a metal foil by coating the varnish on the metal foil and drying the same.
However, when such a resin is used in the production of a flexible printed circuit base board for the above-mentioned purpose, the following disadvantages are seen:
(1) The coating obtained by application and drying of said resin is, in most cases, poor in mechanical strength, so that it can hardly serve as a supporting film for a flexible printed circuit and cannot withstand the mechanical stress in the subsequent step of processing the printed circuit base board and in the actual use thereof.
(2) Thermoplastic resins which can be easily formed into a film by the solution casting method are usually unsatisfactory in heat-resistance in the processing of the flexible printed circuit base board and in the actual mounting of the resulting printed circuit. In order to overcome these disadvantages, there has recently been proposed a method for manufacturing the flexible printed circuit boards by said solution casting method using a heat-resistant resin having a heterocyclic ring such as polyimide, polyamide-imide, polyesterimide, etc. The use of such a resin results in a film having an excellent heat resistance, but the adhesion of the film to the metal foil becomes unsatisfactory because of shortage of polar groups. Also, when applying such a resin on a metal foil surface and curing it by drying, there takes place cure-shrinkage to cause a great curling of the base board toward the opposite side of the metal foil, that is, the resin film side, whereby the practicability of the printed circuit board becomes unsatisfactory. Such a curling tendency becomes greater by use of resins having a higher molecular weight. If no high molecular weight resin is used, the coating becomes weak in molecular bond, poor in film-formability, inferior in mechanical strength, and brittle. Thus, in the case of using a heat-resistant resin containing a heterocyclic ring, it is impossible to obtain a coating good in all of the film-formability, adhesion to metal foil and curling property, that is, well-balanced in properties, and the resulting laminate lacks practicability.
When only a heat-resistant resin having a heterocyclic ring is used, the adhesion to metal foil is poor because of shortage of polar groups although the heat resistance is excellent. There also appear curls and creases due to cure-shrinkage. Therefore, the present inventors have made extensive studies for overcoming these difficulties without diminishing the heat-resistance of said resin to find that all of these problems can be solved entirely by reacting said resin with an epoxy resin.